MB → motherboard
Since no one mentioned this being an OS issue, try a live boot with a Linux distro. If you cannot achieve the ~2.5Gbps speed there then the issue lies with the adapter itself.
Usually you have to enroll in order to get any build updates. If you're not signed into a Microsoft Account then there are alternatives like OfflineInsiderEnroll.
Corrected link: HP Z2 Small Form Factor G4 Workstation (6TT78EA): beste prijs - Tweakers
You have the SFF model which is a lower profile chassis. That means that the Noctua cooler you linked won't fit. You'd need something shorter in height like the NH-L9x65. But I'm not entirely sure your...